The present invention relates to a brake member for automobiles such as a brake drum and a brake rotor, and also to a method of manufacturing the same.
Widely used as brake members for automobiles are brake drums and brake rotors. Typical examples of the brake drums and the brake rotors are shown in FIGS. 1 and 2. In FIG. 1, a drum 1 is fixed to a hub 2 with a plurality of bolts 3 and nuts 4. The same is true of FIG. 2, in which a rotor 21 is fixed to a hub 22 with a plurality of bolts 23 and nuts 24. The drum 1 and the rotor 21 are required to have good braking properties such as wear resistance, damping capacity and heat conductivity. Accordingly, they are usually made of flaky graphite cast iron such as FC25. On the other hand, since the hubs 2, 22 are required to have good mechanical strength and toughness, they are usually made of spheroidal graphite cast iron such as FCD50.
Thus, conventionally used for brake members are a combination of a drum or a rotor made of flaky graphite cast iron and a hub made of spheroidal graphite cast iron, fixed together with bolts.
Various attempts have recently been made to provide an integral brake member consisting of a disc or rotor portion with good wear and damping properties and a hub portion with good mechanical properties.
Japanese Patent Laid-Open No. 56-93851 discloses a disc brake rotor consisting of a disc portion made of flaky graphite cast iron and a boss portion made of spheroidal graphite cast iron. This disc brake rotor is produced from a single melt having a graphite flake cast iron composition. A spheroidizing material such as Mg, Ca and their alloys is placed in a cavity of a mold at such positions that the melt poured into the cavity can flow while in contact therewith into an area corresponding to the boss portion of the disc brake rotor. Therefore, the boss portion has a microstructure of spheroidal graphite cast iron.
Japanese Patent Laid-Open No. 57-9852 discloses a cast iron product such as a disc brake rotor and a brake drum for automobiles having spheroidal graphite distributed in a particular portion and compacted/vermicular graphite in the other portion. In this case too, a spheroidizing material is placed in a mold cavity near a portion which is to be formed with compound/vermicular graphite cast iron. Since the compacted/vermicular graphite cast iron has good mechanical properties, it is used for forming a hub portion of the disc brake rotor.
In both of the above prior art, however, the use of a spheroidizing material does not always ensure the formation of spheroidal graphite cast iron or C/V graphite cast iron in the desired portion, because mere contact of the spheroidizing material with the cast iron melt flowing thereover is not enough to provide full mixing thereof. Thus, mechanical strength may vary widely for the hub portions produced by contact of the spheroidizing material with the cast iron melt in a mold cavity, which means that these technologies using the spheroidizing material are unable to provide high-quality brake drums and rotors for sure.
Japanese Patent Laid-Open No. 59-45073 discloses a wheel hub integrally provided with a brake drum. The hub portion is formed from a spheroidal graphite cast iron melt while the drum portion is from a gray cast iron melt. Since this technology does not use a spheroidizing material placed in a mold cavity, the resulting wheel hub appears to have a good quality. However, to fuse the two portions integrally in a mold cavity, two melts having different compositions should be poured into the cavity through two runners, and the mold should have a means for detecting whether or not a first melt fills a predetermined area in the cavity. Accordingly, a mold for providing this wheel hub inevitably has a complicated structure, and a method of producing the wheel hub by such mold is also inevitably difficult and costly.
And recently, consumers have been requesting increasingly higher braking characteristics. Among other things, good damping capacity for preventing a braking noise, so-called "squealing" is strongly desired. By recent demand of consumers, flaky graphite cast iron such as FC25 is found lacking with respect to damping capacity.